Conventionally, there exist computer systems which provide a host computer with large-scale data storage services. Among such systems, a system is known which comprises a host computer, a storage server to which the host computer is connected, and a management apparatus for the storage server.
The storage server manages a plurality of hard disks using a RAID (Redundant Array of Independent/Inexpensive Disks) system. The storage server also virtualizes a physical storage area of a multiplicity of hard disks, and provides this physical storage area as logical volumes to the host computer. The host computer accesses the logical volumes and requests data reads and data writes.
One example of this type of virtualization technology is known as ‘Thin Provisioning.’ The storage server configures a virtual volume, which has no physical storage area, for the host computer. The storage server sequentially allocates storage area to the virtual volume in step with the host computer write-accessing the virtual volume.
Therefore this technology is effective in enabling storage resources to be effectively utilized in comparison with systems in which a large-capacity storage area is allocated to the logical volumes from the outset. This technology appears in Japanese Unexamined Patent Publication Nos. 2003-15915 and 2006-338341.
Means for providing storage area to the virtual volume comprises means obtained by placing a plurality of logical volumes each assigned real storage area into logical groups called ‘pools.’ The plurality of logical volumes are called ‘pool volumes.’ When the virtual volumes are write-accessed by the host computer, a corresponding storage area of the pool volume is assigned to the virtual volume access destination.
The storage server is capable of saving write data from the host computer by mapping an area of access by the host computer to the virtual volume to an area in which storage capacity has been allocated to the pool volume.
Meanwhile, in order to improve efficiency of investment in the storage server, a storage server is required in which used storage media (physical devices) of a plurality of types are prepared, and which is capable of defining data values and storing data by associating the values of the data with the storage media. Therefore, U.S. Patent Publication No. 2005/055603 discloses the dynamic re-allocation of suitable storage media to pages, which are the units in which capacity is allocated, in response to input/output access to the pages.